Currently, digital television DSS/DVB signals are transmitted in an MPEG format. In the MPEG format, the signals are transmitted in the form of data packets, with each packet including a useful signal or data and a header signal or data containing information about the contents of the useful signal or data.
Such a digital signal generally consists of packets of various types, such as audio data packets, video data packets and packets containing information pertaining to the program transmitted.
In general, the decoding of such digital signals takes place in a decoder in a reception station, which identifies and selects, in the incoming signals, desired audio and video data packets and then decodes these packets to form data trains, of the audio and video types, respectively. The audio data trains are decoded by means of an audio decoder for forming an analog acoustic signal. Similarly, the video data trains are used to form an image as well as chrominance and luminance signals.
There are known devices for the identification of packets. These devices extract a piece of data from each header signal, which is representative of the type of the corresponding useful signal. They include a means for storing reference data in a memory, at addresses each corresponding to one packet type, and a means for comparing the piece of data extracted from each header signal with said reference data (stored in the memory) then delivering to a data processing unit for further processing, an address signal indicating the nature of the corresponding packet. The processing unit then selects the identified packets for decoding and for forming corresponding data trains.
For this type of identification device, the comparison between the piece of data extracted from the header signal and the reference data stored in memory is conducted successively; that is, at a transition of a synchronization clock, an extracted piece of data is compared to a reference piece of data.
However, since the transmission rate of the packets is very high, the packet being identified must be stored, for example, in a memory of the FIFO type, associated with a piloting circuit and then further processed by the data processing unit.
Consequently, this type of identification device is relatively slow, requires a large number of components, and requires a large amount of memory and/or local storage buffers.
International standardization committees have been working on the specification of the coding methods and transmission formats for several compression algorithms to facilitate world wide interchange of digitally encoded audiovisual data. The Joint Photographic experts Group (JPEG) of the International Standards Organization (ISO) specified an algorithm for compression of still images. The ITU (formerly CCITT) proposed the H.261 standard for video telephony and video conference. The Motion Pictures Experts Group (MPEG) of ISO specified a first standard, MPEG-1, which is used for interactive video and provides a picture quality comparable to VCR quality. MPEG has also specified a second standard, MPEG-2, which provides audiovisual quality of both broadcast TV and HDTV. Because of the wide field of applications MPEG-2 is a family of standards with different profiles and levels.
The JPEG coding scheme could be in principal also used for coding of images sequences, sometimes described as motion JPEG. However, this intraframe coding is not very efficient because the redundancy between successive frames is not exploited. The redundancy between succeeding frames can be reduced by predictive coding. The simplest predictive coding is differential interframe coding where the difference between a current pixel of the present frame and the corresponding pixel of the previous frame is quantized, coded and transmitted. To perform such interframe prediction a frame memory for storing one or more frames is required to allow for this pixel by pixel comparison. Higher efficiency than the simple differential interframe coding can be achieved by a combination of discrete cosine transform (DCT) and interframe prediction. For so-called hybrid coding the interframe difference, which is similar to JPEG, is obtained, DCT coded and then transmitted. In order to have the same prediction at both the receiver and transmitter the decoder is incorporated into the coder. This results in a special feedback structure at the transmitter which avoids coder-decoder divergence.
Variable word length coding results in a variable bit rate which depends on image content, sequence change, etc. Transmission of the coded information over a constant rate channel requires a FIFO buffer at the output to smooth the data rate. The average video rate has to be adjusted to the constant channel rate. This is performed by controlling the quantizer according to the buffer content. If the buffer is nearly full, the quantization is made more severe and thus the coded bitrate is reduced. Conversely, if the buffer is nearly empty, the quantization is relaxed.
In general, the MPEG coding use a special predictive coding strategy. The coding starts with a frame which is not differentially coded; it is called an Intraframe (I). Then prediction is performed for coding one frame out of every M frames. This allows computation of a series of predicted frames (P), while "skipping" M-1 frames between coded frames. Finally, the "skipped" frames are coded in either a forward prediction mode, backward prediction mode, or bi-directional prediction mode. These frames are called bi-directionally interpolated (B) frames. The most efficient prediction mode, in terms of bitrate, is determined by the encoder and its selected mode is associated with the coded data. Thus the decoder can perform the necessary operations in order to reconstruct the image sequence. A main difference between MPEG-1 and MPEG-2 is that MPEG-1 has been optimized for non-interlaced (progressive) format while MPEG-2 is a generic standard for both interlaced and progressive formats. Thus, MPEG-2 includes more sophisticated prediction schemes.
In more detail, motion pictures are provided at thirty frames per second to create the illusion of continuous motion. Since each picture is made up of thousands of pixels, the amount of storage necessary for storing even a short motion sequence is enormous. As higher and higher definitions are desired, the number of pixels in each picture grows also. This means that the frame memory used to store each picture for interframe prediction also grows; current MPEG systems use about 16 megabits (MB) of reference memory for this function. Fortunately, lossy compression techniques have been developed to achieve very high data compression without loss of perceived picture quality by taking advantage of special properties of the human visual system. (A lossy compression technique involves discarding information not essential to achieve the target picture quality to the human visual system). An MPEG decoder is then required to reconstruct in real time or nearly real time every pixel of the stored motion sequence; current MPEG decoders use at least about 16 MB of frame memory for reconstruction of frames using the encoded interframe prediction data.
The MPEG standard specifies both the coded digital representation of video signal for the storage media, and the method for decoding to achieve compatibility between compression and decompression equipment. The standard supports normal speed playback, as well as other play modes of color motion pictures, and reproduction of still pictures. The standard covers the common 525- and 625-line television, personal computer and workstation display formats. The MPEG-1 standard is intended for equipment supporting continuous transfer rate of up to 1.5 Mbits per second, such as compact disks, digital audio tapes, or magnetic hard disks. The MPEG-2 standard supports bit rates from 4 Mbits/sec (Mbits) to 15 Mbits and is targeted for equipment that complies with the International Radio Consultative Committee (CCIR) recommendation 601 (CCIR-601). The MPEG standard is intended to support picture frames at a rate between 24 Hz and 30 Hz. ISO-11171 entitled Coding for Moving Pictures and Associated Audio for digital storage medium at 1.5 Mbit/s, provides the details of the MPEG-1 standard. ISO-13838 entitled Generic Coding of Moving Pictures and Associated Audio provides the details of the MPEG-2 standard.
Under the MPEG standard, the picture frame is divided into a series of "Macroblock slices" (MBS), each MBS containing a number of picture areas (called "macroblocks") each covering an area of 16.times.16 pixels. Each of these picture areas is represented by one or more 8.times.8 matrices which elements are the spatial luminance and chrominance values. In one representation (4:2:2) of the macroblock, a luminance value (Y type) is provided for every pixel in the 16.times.16 pixels picture area (in four 8.times.8 "Y" matrices), and chrominance values of the U and V (i.e., blue and red chrominance) types, each covering the same 16.times.16 picture area, are respectively provided in two 8.times.8 "U" and two 8.times.8 "V" matrices. That is, each 8.times.8 U or V matrix covers an area of 8.times.16 pixels. In another representation (4:2:0), a luminance value is provided for every pixel in the 16.times.16 pixels picture area, and one 8.times.8 matrix for each of the U and V types is provided to represent the chrominance values of the 16.times.16 pixels picture area. A group of four continuous pixels in a 2.times.2 configuration is called a "quad pixel"; hence, the macroblock can also be thought of as comprising 64 quad pixels in an 8.times.8 configuration.
The MPEG standard adopts a model of compression and decompression. Initially, interframe redundancy is first removed from the color motion picture frames. To achieve interframe redundancy removal, each frame is designated either "intra" "predicted" or "interpolated" for coding purpose. Intraframes are least frequently provided, the predicted frames are provided more frequently than the intraframes, and all the remaining frames are interpolated frames. The values of every pixel in an intraframe ("I-picture") is independently provided. In a prediction frame ("P-picture"), only the incremental changes in pixel values from the last I-picture or P-picture are coded. In an interpolation frame ("B-picture"), the pixel values are coded with respect to both an earlier frame and a later frame. Again, large (16 MB) frame or reference memories are required to store frames of video to allow for this type of coding.
The MPEG standard does not require frames to be stored in strict time sequence, so that the intraframe from which a predicted frame is coded can be provided in the picture sequence either earlier or later in time from the predicted frame. By coding frames incrementally, using predicted and interpolated frames, much interframe redundancy can be eliminated which results in tremendous savings in storage requirements. Further, motion of an entire macroblock can be coded by a motion vector, rather than at the pixel level, thereby providing further data compression.
The next steps in compression under the MPEG standard remove intraframe redundancy. In the first step, a 2-dimensional discrete cosine transform (DCT) is performed on each of the 8.times.8 values matrices to map the spatial luminance or chrominance values into the frequency domain.
Next, a process called "quantization" weights each element of the 8.times.8 matrix in accordance with its chrominance or luminance type and its frequency. In an I-picture, the quantization weights are intended to reduce to one many high frequency components to which the human eye is not sensitive. In P- and B-pictures, which contain mostly higher frequency components, the weights are not related to visual perception. Having created many zero elements in the 8.times.8 matrix, each matrix can now be represented without information loss as an ordered list of a "DC" value, and alternating pairs of a non-zero "AC" value and a length of zero elements following the non-zero value. The list is ordered such that the elements of the matrix are presented as if the matrix is read in a zigzag manner (i.e., the elements of a matrix A are read in the order A00, A01, A10, A20, A11, A02, etc.). The representation is space efficient because zero elements are not represented individually.
Finally, an entropy encoding scheme is used to further compress the representations of the DC block coefficients and the AC value-run length pairs using variable length codes. Under the entropy encoding scheme, the more frequently occurring symbols are represented by shorter codes. Further efficiency in storage is thereby achieved.
In decompression, under MPEG, the processes of entropy encoding, quantization and DCT are reversed. The final step, called "absolute pixel generation", provides the actual pixels for reproduction, in accordance to the play mode (forward, reverse, slow motion e.g.), and the physical dimensions and attributes of the display used. Again, large (16 MB) frame or reference memories are required to store frames of video to allow for this type of reproduction.
Since the steps involved in compression (coding) and decompression (decoding), such as illustrated for the MPEG standard discussed above, are very computationally intensive and require large amounts of memory, for such a compression scheme to be practical and widely accepted, the decompression processor must be designed to provide decompression in real time, and allow economical implementation using today's computer or integrated circuit technology.
The purpose of the present invention is to overcome these short-comings and drawbacks. Improvements in circuits, integrated circuit devices, computer systems of all types, and methods to address all the just-mentioned challenges, among others, are desirable, as described herein.